The present invention relates to submersible pumps and particularly, to submersible pumps for radioactive fluids which present difficult problems both in terms of storage and conveying. The pump of the present invention is of the type that can be submersed into the fluid medium and which can be driven through a magnetic coupling mounted in a housing that is isolated from the motor. A series of shaft sections are provided so that the housing for the fluid displacement member can be submersed to any desired depth in an underground storage tank by coupling the appropriate number of shaft sections together.
With the increasing use of nuclear power generators, storage tanks for the treatment and intermediate storage of radioactive fluids and solutions will become increasingly commonplace. In order to shield the environment and persons working in the vicinity of the stored fluids against radiation, the tanks must be mounted in thick-walled concrete cells. It has been proposed that several such tanks be collected in one storage area with interconnecting plumbing establishing fluid communication between such tanks to facilitate filling and discharge via a conduit system which, itself, must be shielded to prevent radiation leaks. Because of the high level of radiation that can exist in such facilities, it is impossible after filling of the storage tanks, to enter the storage tank cell for making repairs or the like. Repair and maintenance work in areas exposed to radioactive fluid and solutions can be carried out only after thorough flushing of the area including the conduits and pipe fittings. Also, in these systems, it must be possible to fill and discharge the tanks with rigorous safeguards against leakages. Similarly, it is extremely important that in case of a leak developing in a tank, it be possible to quickly transfer the contents of the damaged tank to a reserve tank.
A number of criteria relating to the design and the construction of tanks of this type must be employed which place limitations on the types of liquid pumps that can be employed. For example, many designs require that the tanks be buried in the ground so that any pump that is employed must be able to lift the liquid 10 or more meters in height, while the liquid itself is at a temperature of 60.degree. C. or higher. As a result, the use of self-priming rotary pumps located at ground level becomes impractical. In previously known installations, where pumps are employed for conveying radioactive solutions, steam ejectors or airlift types of pumps have proven useful. However, steam ejectors have the disadvantage that the liquid being pumped is heated by the steam thereby increasing its volume by the amount of the condensed steam. Nevertheless, due to the reliability and simplicity of such arrangements, steam jets have been used as a relatively safe conveying mechanism. On the other hand, pump systems employing air under pressure have not proven as useful when large quantities of liquid must be conveyed, particularly where a vacuum system must be employed to assure that the liquid is pumped to the desired level. Additionally, airlift systems have the disadvantage that radioactive exhaust air develops in unacceptable quantities, thus further complicating the equipment that must be employed. As another solution, compact submersible pumps, where the driving motor is also submersed into the solution to be pumped, have been proposed but use of such has resulted in considerable difficulties because of the requirement of employing expensive radiation resistant seals and electrical insulation materials.
It has also been proposed to use pumps of the type which are lowered into the tank to just above the maximum level of the liquid in the tank. This type of arrangement includes a self-priming pump with a compact receiver for the liquid. One of the drawbacks of this type of arrangement is that the starting operation requires an amount of fluid to be supplied. Moreover, with this type of arrangement, in view of the presently existing possible operating conditions where the liquid may be at or near 60.degree. C. and a 6-meter suction height required, a safe suction of the liquid to be pumped is not always assured.
The pump of the present invention overcomes the foregoing difficulties and provides a very safe pumping system for a radioactive liquid solution contained in a tank. According to the present invention a fluid displacement impeller type pump is disposed at the lowest point in the storage tank so that the required suction will be reduced to a minimum or zero at the start-up of a pumping operation. Further, priming fluid will no longer be necessary and the pump will be able to operate even in the case of a blocked conduit in the pressure line without causing damage to the pump. The fluid being pumped will be safely encased at all times in an exhaust conduit which can be easily drained by a built-in drain hole which leads back to the storage tank. In addition, the parts of the pump system that may need servicing will be readily accessible and can be assembled and disassembled very rapidly in a wide range of storage tanks having different capacities.
Specifically, according to the present invention, there is provided a submersible pump which consists of a fluid displacement element mounted in a housing which is to be totally immersed in the fluid to be pumped. The housing has a magnetic coupling so that the driving element is isolated from the fluid displacement impeller which contacts the radioactive solution. The drive motor and lubricant supply tank are isolated from the radioactive solution and may be at ground level, for example, where the tank is buried. A shaft coupling system for establishing connection between the motor and fluid displacement impeller is supplied which assures the insulation of the drive shaft from the radioactive solution and which also provides a safe connection for the outlet from the impeller housing through which the liquid is to be pumped. In this arrangement, a plurality of shaft sections are provided each of which includes a tube for the drive shaft, a pipe for the liquid being pumped and a lubricant conduit. These three tubes or conduits are joined together at their opposite ends by coupling means. The coupling means of each shaft section includes a coupling flange member which will afford rapid and accurate coupling of the shaft sections together so that the housing containing the fluid displacement impeller can be lowered to any desired depth in a tank.
By making the shaft sections of a manageable length, for example, 0.5 to 2 meters, they can be easily disassembled and replaced in case of damage or a leak occurring along the conveying path.